This invention relates generally to a partially porous coated prosthesis with interlocking fixation and the provision of stress shielding reduction.
Prosthesis fixation by growth of bone into porous-coated implants is well known in the art as an alternative fixation means to the use and known limitations of various other fixation means, for example screws, spikes, acrylic cement, etc. As is further known in the art with regard to such porous-coated implants, bone has been shown to grow into the voids of porous coated prostheses producing a mechanically effective biological bond. The interlocking of viable bone with the porous prosthetic surface has been found to provide a three-dimensional fixation which resists both tensile and shear stress at the implant-bone interface.
A porous coating can also be employed to improve the bonding ability of bone cement. Where cement is used in apposition to a smooth prosthetic surface, as is often the case, one obtains a relatively weak bond at the prosthesis-cement interface. Properly applied bone cement, however, produces a strong cement to bone interface wherein the cement is forced into the interstices of the bone. The strength of fixation is limited by the strength of the relatively weak prosthesis to cement interface and does not utilize effectively the strength of the bone to cement interface. The use of a porous surface prosthesis, however, produces interlocking at both the cement-bone and prosthesis-cement interfaces thus producing three-dimensional fixation which is resistant to both tensile and shearing loads.
Stress shielding is a problem that is also known in the porous-coated implant art which stress-shielding is taught in detail in an article entitled, "POROUS INGROWTH FIXATION OF THE FEMORAL COMPONENT IN A CANINE SURFACE REPLACEMENT OF THE HIP," by Anthony K. Hedley et al., published March 1982, No. 163, Clinical Orthopaedics & Related Research. As taught in detail in the Hedley et al. article, and as shown diagrammatically in FIG. 1 of the drawings, which FIG. is taken from FIG. 4 of the Hedley article, it is known and illustrated that upon the entire interior surface 12 of a femoral hip surface replacement component 10 being porous-coated, and upon the prosthesis being implanted in the head of the femur 14, bone ingrowth will occur at the interface between the bone and the porous coated prosthesis stem 16 providing good fixation but it has been found, as illustrated, that the load or stress will be transferred to this interface, and the interface between the head of the femur and the interior surface 12 of the prosthesis will be shielded from the load or stress which is transferred to the interface between the bone and the porous coated stem 16 and this stress-shielding has been found to cause resorption and hypertrophy at the head of the bone causing the shown void between the head of the femur 14 and the interior surface 12 of the prosthesis 10. As is further known to those skilled in the art, interior wall fixation and bone ingrowth between the head of the femur 14 and the interior surface 12 of the prosthesis 10 is more desirable than fixation and bone ingrowth between the femur 14 and porous-coated surface of the stem 16 because it provides a more uniform loading between the prosthesis and the femur and a more uniform transfer of stress from the prosthesis to the resected femoral head. As is also taught in the Hedley et al. article, in FIG. 6 thereof, and as shown in FIG. 2 of the drawings taken from such FIG. 6, the stress-shielding problem associated with the porous coating of the surface of the stem 16 of the prosthesis 10 of FIG. 1 can be eliminated by eliminating the stem thereby producing direct bone ingrowth between the resected head of the femur 14 and the entire interior surface 12 of the prosthesis 10 achieving the above-noted preferable fixation between the bone and the interior wall of the prosthesis with its attendant uniform loading and stress transfer.
Although the use of a stem is not necessary for the purpose of providing axial fixation in a bone capping type prosthesis as described in Hedley, in other applications a stem is important in providing initial fixation in order to provide fixation means while bone ingrowth occurs in order to minimize patient or joint immobilization for an undesirably lengthy period of time or to help prevent motion between the bone and prosthesis interfaces which can occur as a result of patient movement wherein such motion can prevent ingrowth by repeated rupture of early bone ingrowth with resultant fixation failure.
The use of stems also can provide alignment capabilities improving the positional accuracy of placement of the prosthesis. Further, stems can provide fixation which augments resistance to the joint reaction loads and can be useful in preventing possible fractures.
Stress shielding can also occur with cemented prostheses where the cement provides the strong interlocking between implant and bone. This phenomenon has been seen clinically on well fixtured hip stem prostheses in cases where there is excellent distal fixation of the stem. This situation produces load transfer from prosthesis to bone at the distal aspect of the prosthesis leaving the bone proximal to this region unloaded thereby resulting in stress shielding and resorption of the proximal bone.
The auxiliary stem functions, including alignment, initial fixation fracture prevention, and load augmentation can all be considered as providing secondary fixation since all provide at some time a load transfer function. The alignment function provides load transfer between prosthesis and bone during implantation wherein this loading controls positioning of the component during implantation. Preventing of fracture also involves load transfer preventing overloading of predetermined regions of bone.
For a prosthesis made of material, such as metal or ceramic, substantially stiffer than bone, and where firm fixation and substantial load transfer occurs in bony regions remote from the load application surface, loading of the bony regions nearer the load application surface is substantially reduced shielding the nearer bone against stress. On the other hand, if firm fixation and substantial load transfer occurs in bony regions near the load application surface, loading of bone further from the load application surface is not substantially reduced and therefore such bone is not substantially protected against stress. Thus, for relatively stiff prostheses it is desirable to design fixation so as to minimize load transfer regions of bone away from the load application surface in order to minimize bone resorption of bone nearer the load application surface.
Accordingly, it is a primary object of the present invention to provide an improved prosthesis providing interlocking fixation between the prosthesis and a bone and reducing the above-noted stress shielding problem.